Direct current operable are discharge devices generally are double-ended and require an anode electrode having a mass considerably larger than the cathode owing to the greater power input which must be dissipated. Since the anode fall voltage is generally much less than the fall of the cathode, this disproportionate anode input results principally from a combination of three factors, viz.: (a.) the electrons give energy to the anode upon entering as they drop into the conduction band; (b.) the fractional electron emission cools the cathode by the inverse process of (a.); and (c.) the fractional electron current component at the anode being higher than the cqrresponding fractional ion current at the cathode offsets much of the electrode fall difference which would otherwise tend to favor preferential cathode heating.
These conditions have led the prior art to employ relatively large anodes and small, pointed cathodes to balance electrode heating. This solution, however, leads to discharge instability, which occurs when the cathode temperature falls too low for favorable thermionic emission.